Ultra-high-energy cosmic rays

In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray particle with a kinetic energy greater than 7018100000000000000♠1×10¹⁸ eV, far beyond both the rest mass and energies typical of other cosmic ray particles.

An extreme-energy cosmic ray (EECR) is an UHECR with energy exceeding 7000801088243500000♠5×10¹⁹ eV (about 8 joule), the so-called Greisen–Zatsepin–Kuzmin limit (GZK limit). This limit should be the maximum energy of cosmic ray protons that have traveled long distances (about 160 million light years), since higher-energy protons would have lost energy over that distance due to scattering from photons in the cosmic microwave background (CMB). It follows that EECR could not be survivors from the early universe, but are cosmologically "young", emitted somewhere in the Local Supercluster by some unknown physical process. If an EECR is not a proton, but a nucleus with ${\displaystyle A}$ nucleons, then the GZK limit applies to its nucleons, which carry only a fraction ${\displaystyle 1/A}$ of the total energy of the nucleus. For an iron nucleus, the corresponding limit would be 7002448609416360000♠2.8×10²¹ eV.

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